It has been prior art for a long time to use elastic, electrically conductive seals for sealing electromagnetically-screened multi-component housing, etc. The seals securely close the gaps between the housing parts and provide a good electrical contact between the (electrically-screening) housing parts. Such seals consist of tubes made of wire mesh or metallic fleeces, or frequently an elastomer mixed with conductive fillers (carbon particles, copper or silver particles, metallically-coated spheres, silver-coated particles, etc.). The elastomer is used in an unhardened state to form a corresponding sealing bead or strip. The formed bead is then hardened (by heat, radiation, humidity, etc.). In U.S. Pat. No. 4 659,869, a screening seal is described where a sealing element made of a conductive elastomer is extruded in the form of a tube and then connected to an elongated clamping strip made of metal. This type of seal is very involved to manufacture due to the clamping strip. In addition, it can only be used for limited instances because there must be a straight, strip-like edge of sheet metal on the housing on which to clamp the clamping bead. In particular, this technique cannot be used to create an annular closed seal with complicated dimensions.
Another type of screening seal is described in the initially-cited EP A2 0 241 192. A flat substrate in the form of an annular strip is printed on one or both sides by screen printing with one or more sealing beads made of an electrically-conductive elastomer. Silk screen printing makes it easy to create different seal shapes in contrast to the above-cited case. The use of silk screen printing also has disadvantages, however. Each seal shape must be created in correspondingly shaped screen. Although this does not provide a limitation for large-scale series production, small-scale series are not economical due to the frequently changing shapes. Furthermore, silk screen printing can only create seal beads that have a uniform, essentially rectangular cross-section. Variations of the height or cross-section are nearly impossible with silk screen printing. Finally, it is impossible to change the composition of the seal material in relation to the site to e.g. adjust the reaction to pressure, etc. of the seal bead.
Another sealing technique for screening seals that is not limited as above has been suggested by the applicant in an earlier patent application (EP A10 629 114). In this sealing technique, the seal bead is applied directly to a surface of the housing to be sealed. The sealant is applied as a pasty, electrically-conductive elastomer by an applicator with a nozzle in the form of a bead. The applicator follows the shape of the seal like a robot that is controlled along several axes. With this technique (termed "form-in-place"), the sealing bead is hardened on the sealing surface as shaped by and applied by the nozzle. Suitable sealing materials for this process are e.g. disclosed in EP A10 643 551 and EP Al 0 643 552.
The form-in-place technique is very flexible to use, can be economically used for large-scale and small-scale series, and can easily create seals for very small housings with complicated shapes. Additionally by controlling the supply of the sealing material to the nozzle, the composition and cross-section of the sealing bead can be easily varied in relation to the site. Finally, it is easy to create complicated cross-sections of the sealing bead and hence special characteristics of the seal by applying several parallel individual beads either sequentially or with multiple nozzles.
With the form-in-place technique, at least the housing part (or the printed circuit board) that has the surface to which the seal is applied must be at the site of manufacture for the seal bead to be applied. However, it would be desirable to use the indisputable advantages of the form-in-place technique for those instances in which the parts to be sealed are not at the site of manufacture.